The present invention relates to an electrophotographic material which comprises an electrically conductive support, a photoconductor layer applied thereto formed of organic or inorganic photoconductive substances, and a protective layer covering the photoconductor layer. The invention also relates to a process for producing such an electrophotographic material.
Electrophotographic recording materials provided with organic photoconductor layers are disclosed in U.S. Pat. No. 3,037,861, U.S. Pat. No. 3,307,940, U.S. Pat. No. 3,169,060 and in British Pat. No. 1,178,592. Materials with inorganic photoconductors, such as selenium layers, are described in U.S. Pat. No. 3,434,832.
The production of dry copies according to the so-called toner image transfer process requires thorough cleaning of the photoconductor layer after each transfer. Cleaning is usually done by brushing or wiping the photoconductor with brushes or cloths. In transfer processes using liquid developers, the cleaning action is increased by additionally applying a liquid cleaning agent. Such cleaning operations produce mechanical stresses on the photoconductor layer and may thus damage the layer and lead to the occurrence of wear.
In addition to these cleaning operations, other detrimental influences act on the photoconductor layer. For example, the layer is subject to the action of a developer composed of carrier and toner particles and, in the case of liquid development, also to the action of the developing liquid which must not in any event cause an incipient dissolution or swelling of the photoconductor layer. In addition, the ionized air generated in the charging station may damage the photoconductor layer by causing cracking or embrittlement phenomena.
Generally, these influences will adversely affect the photoconductor layer and, consequently, shorten its useful life.
U.S. Pat. Nos. 2,860,048 and 3,617,265 and German Offenlegungsschriften 15 72 368 and 24 52 623 disclose protecting photoconductor layers from the above-described influences by providing them with a protective layer. However, comparatively high temperatures are required during or after the coating procedure for applying these coverings. As a result, the dimensional stability of the support may suffer, for example, if the support is a plastic film. Furthermore, the electric properties of the applied protective coatings do not meet the high standards needed with respect to sufficient discharge of the photoconductor layer when exposed to a light image. The undesirably high residual charge which remains leads to copies of reduced contrast which have a darkened background. Some of the covering described in the prior art also have an undesirable tendency towards turbidity or haziness.
Japanese published patent application Ser. No. 75-54,441 discloses using protective layers of polymerized cinnamic esters for electrophotographic materials. Such materials are, however, expensive special manufactures which are not well suited for normal technical application.